1. Field of the Invention
The present invention relates to a heat exchanger, and more particularly, to a heat exchanger using carbon dioxide as a refrigerant.
2. Description of the Related Art
In general, a heat exchanger is an apparatus for exchanging heat by transferring heat of a fluid at a high temperature to a fluid at a low temperature through a wall surface. A freon-based refrigerant has mainly been used as a refrigerant of an air conditioning system having a heat exchanger thus far. However, as the freon-based refrigerant is recognized as a major factor of global warming, the use thereof is gradually restricted. Under the above circumferences, studies about carbon dioxide as a next generation refrigerant to replace the present freon-based refrigerant is actively being developed.
The carbon dioxide is regarded as an eco-friendly refrigerant because the global warming potential (GWP) thereof is just about {fraction (1/1300)} of R134a that is a typical freon-based refrigerant. In addition, the carbon dioxide has the following merits.
The carbon, dioxide refrigerant has a superior volumetric efficiency because an operational compression ratio is low, and a smaller difference of temperature between air that flows in and the refrigerant out of a heat exchanger than that of the existing refrigerant. Since heat transferring performance is excellent, the efficiency of cooling cycle can be improved. When the temperature of the outside air is as low as in the winter time, since heat can be extracted from the outside air by only a small difference in temperature, the possibility of applying the carbon dioxide refrigerant to a heat pump system is very high.
Also, since the volumetric cooling capability (latent heat of vaporizationxc3x97gas density) of carbon dioxide is 7 or 8 times of R134a that is the existing refrigerant, the volume size of a compressor can be greatly reduced. Since the surface tension thereof is low, boiling heat transfer is superior. Since the specific heat at constant pressure is great and a fluid viscosity is low, a heat transfer performance is superior. Thus, the carbon dioxide refrigerant has superior thermodynamic features as a refrigerant.
Also, in view of the cooling cycle, since the operational pressure is very high such that it is 10 times high at an evaporator side and 6-8 times high at a gas cooler (an existing condenser) side compared to the conventional refrigerant, a loss due to a pressure drop in the refrigerant inside the heat exchanger is relatively low compared to the existing refrigerant, so that a micro channel heat exchange tube exhibiting superior heat transfer performance with great pressure drop can be used.
However, since the cooling cycle of carbon dioxide is a transcritical pressure cycle, not only a vaporization pressure but also a gas-cooling pressure is high by 6-8 times compared to the existing cycle. Thus, in order to use carbon dioxide as a refrigerant, evaporator and condenser presently being used should be redesigned to endure such a high pressure.
That is, a laminate type evaporator among the conventional evaporators for cars cannot use carbon dioxide as a refrigerant because it cannot endure a high pressure. A parallel flow type condenser among the conventional condensers for cars needs to be redesigned so that it can be used as a heat exchanger using carbon dioxide as a refrigerant.
Furthermore, the parallel flow type condenser is of a single slab type designed to have one tube row and adopts a multi-pass method of a single slab in which the flow path of the refrigerant is formed in a multi-pass form by adding a plurality of baffles to improve performance. The multi-pass method exhibits a superior distribution of the refrigerant inside the heat exchanger. However, when the refrigerant is in gas cooling, the temperature of the carbon dioxide refrigerant continuously decreases without a condensing process inside the heat exchanger. Accordingly, the deviation of temperature in the whole heat exchanger becomes serious, so that a self heat flow along the surface of the heat exchanger is generated. This flow of heat prevents heat exchanging between the refrigerant and the air coming from the outside and consequently heat transfer performance is deteriorated.
In the meantime, a multi-slab method in which a plurality of tube rows are arranged through which the refrigerant passes to perform heat exchanging, unlike the multi-pass method, can block the heat flow on the multi-pass method, so that it is effective than the multi-pass method using carbon dioxide as a refrigerant.
However, in the heat exchanger in the multi-slap method, pipes to connect each slab should be installed, which is a weak structure to a high pressure. Also, the distribution of the refrigerant in the heat exchanger may be slightly lowered compared to the multi-pass method.
Conventionally, a serpentine type heat exchanger having an increased thickness has been used as a heat exchanger to endure a high operational pressure without considering a feature of the carbon oxide refrigerant. However, such a serpentine heat exchanger exhibits a great pressure drop and an irregular distribution of the refrigerant in the tubes, so that heat transfer performance is deteriorated while the manufacturing cost increases.
Also, in a heat exchanger used as a gas cooler having the same function as a condenser, the temperature of the refrigerant in the heat exchanger decreases due to the heat transfer with the outside air so that the specific volume of the carbon dioxide refrigerant decreases. In the case of the carbon dioxide refrigerant, the difference in specific volume at a heat exchanger is very great, so that the specific volume of carbon dioxide in a refrigerant inlet having a temperature of about 110xc2x0 or more is approximately three times greater than the specific volume of carbon dioxide in a refrigerant outlet having a temperature of about 50xc2x0.
In the heat exchanger using carbon dioxide as a refrigerant showing a great difference in specific volume according to the temperature, maintaining a constant width of a radiating tube is ineffective in view of miniaturization in weight and size of a heat exchanger and a cost for producing parts increases.
In the meantime, in the heat exchanger in the multi-slab method, since independent refrigerant paths of header tanks of the heat exchanger should be connected separately, each path is connected by additional tubes. Thus, to manufacture a heat exchanger having additional tubes requires a lot of work steps to assemble the heat exchanger.
Japanese Patent Publication No. hei 10-206084 discloses a general configuration of a serpentine heat exchanger. The serpentine heat exchanger has a superior structure but may be damaged when the refrigerant acting at a high pressure such as carbon dioxide is used.
Japanese Patent Publication Nos. 2001-201276 and 2001-59687 disclose heat exchangers having an improved pressure resistance feature of a header pipe. These heat exchangers are not far from the serpentine heat exchanger and is limited to be used as the heat exchanger for carbon dioxide.
In addition, Japanese Patent Publication No. hei 11-304378 discloses a heat exchanger for cars in which a radiator and a condenser are integrally formed. However, such a structure is difficult to be adopted, as is, in the heat exchanger for carbon dioxide.
Also, Japanese Patent Publication No. hei 11-351783 discloses a heat exchanger in which an inner post member is further formed at an inner wall of each of header tanks so that a space formed by the inner post members is circular. However, the heat exchanger in which a single tube is connected to two or more spaces formed by the inner post members basically adopts a multi-pass method, which is not appropriate for the heat exchanger for carbon dioxide.
Japanese Patent Publication No. 2000-81294 discloses a heat exchanger by improving the above heat exchangers, in which a single tube is connected to two spaces formed by the inner post members. Since this heat exchanger has a structure in which the refrigerant coming through the tubes are distributed and enter in the two inner spaces, the inner post members can act as a resistance factor to a refrigerant at a high pressure which is exhausted through the tubes.
To solve the above-described problems, it is the first object of the present invention to provide a heat exchanger using a refrigerant, such as carbon dioxide, acting under a high pressure as a heat exchange medium.
It is the second object of the present invention to provide a heat exchanger which can cut the flow of heat in the heat exchanger, in a heat exchanger using a fluid capable of generating flow of heat as the temperature of the fluid continuously decreases in the heat transfer, as a refrigerant, and exhibit a superior pressure resistance feature.
It is the third object of the present invention to provide a heat exchanger in which the distribution of a refrigerant is uniformly formed.
It is the fourth object of the present invention to provide a heat exchanger having a structure in which the refrigerant is smoothly connected in the header pipe.
It is the fifth object of the present invention to provide a heat exchanger having a header pipe which can be adopted in a multi-slab type heat exchanger and can adopt a multi-pass method in the multi-slab type heat exchanger.
It is the sixth object of the present invention to provide a heat exchanger whose weight and size can be reduced when a fluid, such as carbon dioxide, having a great difference in specific volume according to a temperature is used as a refrigerant.
It is the seventh object of the present invention to provide a heat exchanger which can improve thermal characteristics of the refrigerant and simultaneously can be manufactured without greatly modifying the manufacturing equipments for the existing condenser, in a heat exchanger using a fluid, such as carbon dioxide, acting under a high pressure and exhibiting a superior heat transfer feature, as a refrigerant.
To achieve the above objects, there is provided a heat exchanger comprising first and second header pipes arranged a predetermined distance from each other and parallel to each other, each having at least two chambers independently sectioned by a partition wall, a plurality of tubes for separately connecting the chambers of the first and second header pipes, facing each other, wherein the tubes are divided into at least two tube groups, each having a single refrigerant path, a refrigerant inlet pipe formed at the chamber disposed at one end portion of the first header pipe, through which the refrigerant is supplied, a plurality of return holes formed in the partition wall to connect two chambers adjacent to each other, through which the refrigerant sequentially flows the tube groups, and a refrigerant outlet pipe formed at the chamber of one of the first and second header pipes connected to a final tube group of the tube groups along the flow of the refrigerant, through which the refrigerant is exhausted.
It is preferred in the present invention that the refrigerant paths of the tube groups adjacent to each other among the tube groups are opposite to each other.
It is preferred in the present invention that the tube group connected to the chamber where the refrigerant outlet pipe is formed is arranged at an upstream of the flow of air supplied into the heat exchanger.
It is preferred in the present invention that the tube group is formed of a row of the tubes connecting one of the chambers of the first header pipe and one of the chambers of the second header pipe corresponding thereto.
It is preferred in the present invention that at least a baffle for sectioning each chamber is provided at at least two chambers of each of the first and second header pipes, and the row of the tubes connected to the chamber having the baffle are divided into two tube groups with respect to each baffle.
It is preferred in the present invention that the refrigerant inlet pipe and the refrigerant outlet pipe are formed in the same chamber, and that the refrigerant inlet pipe and the refrigerant outlet pipe are formed in different chambers of the first header pipe.
It is preferred in the present invention that the chambers of the first and second header pipes are roughly circular.
It is preferred in the present invention that a thickness of a horizontal section of the partition wall is thicker than a thickness of a horizontal section of the remaining portion of the first and second header pipes.
It is preferred in the present invention that a thickness of a horizontal section of the partition wall is 1.5 through 2.5 times greater than a thickness of a horizontal section of the other portion.
It is preferred in the present invention that each of the return holes is roughly circular, and that each of the return holes is roughly rectangular.
It is preferred in the present invention that each of the first and second header pipes is formed by brazing a header which is extruded or press-processed and has a plurality of slits into which the tubes are inserted and a tank which is extruded or press-processed.
It is preferred in the present invention that the partition wall is integrally formed at at least one of the header and the tank of each of the first and second header pipes.
It is preferred in the present invention that the first and second header pipes comprise at least one caulking coupling portion, and that the caulking coupling portion is provided between at least one of the header and the tank and the partition wall.
It is preferred in the present invention that the partition wall is formed of additional member and brazed to an inner wall of each of the first and second header pipes.
It is preferred in the present invention that thicknesses of the tubes are formed different from one tube group to the other tube group, according to a temperature of the refrigerant flowing through each tube group.
It is preferred in the present invention that the width of each tube of the tube group through which a refrigerant of a high temperature flows is formed to be greater than the width of tube of the tube group through which a refrigerant of a low temperature flows.
It is preferred in the present invention that, when a width of each tube of the tube group through which a refrigerant of a high temperature flows is X and a width of each tube of the tube group through which a refrigerant of a low temperature flows is Y, the X and Y satisfy a relationship that 0.5Xxe2x89xa6Y less than X.
It is preferred in the present invention that each of the tubes comprises a plurality of micro channel tubes, and when a hydraulic diameter of each micro channel tube of the tube group through which a refrigerant of high temperature flows is x and a hydraulic diameter of each micro channel tube of the tube group through which a refrigerant of low temperature flows is y, the x and y satisfy a relationship that 0.5xcexa3xxe2x89xa6xcexa3y less than xcexa3x.
To achieve the above objects, there is provided a heat exchanger comprising, first and second header pipes arranged to be separated a predetermined distance from each other and parallel to each other, a plurality of tubes for connecting the first and second header pipes, wherein the tubes neighboring with each other are connected by a bridge in which a plurality of through holes are formed, a refrigerant inlet pipe formed at one end portion of the first header pipe and through which a refrigerant is supplied to the first header pipe, and a refrigerant outlet pipe formed at one of the first and second header pipes and through which the refrigerant is exhausted.
It is preferred in the present invention that the bridge is formed to be thinner than the tube.
It is preferred in the present invention that each of the first and second header pipes has at least two chambers separated by a partition wall, and the tubes separately connect the chambers of the first and second header pipes facing each other.
It is preferred in the present invention that each of the chambers is divided into at least two spaces extended along a lengthwise direction of each header pipe, and the respective tubes are connected to the spaces of each chamber.